U.S. Pat. No. 3,455,710 describes plaster which can be given water-repellent properties by impregnation with organosiloxanes containing Si-bonded hydrogen atoms (H-siloxanes). This reference also describes the impregnation of mixtures of plaster and calcium hydroxide with H-siloxanes. However, the water-repellent action of the H-siloxanes is unsatisfactory. The H-siloxanes are expensive and evolve hydrogen on contact with water.
U.S. Pat. No. 4,411,702 describes water-repellent shaped articles which are produced from the constituents calcium sulphate dihydrate, slag and calcium hydroxide and in the production of which part of the constituents is treated with a polydimethylsiloxane liquid. However, the water-repellent action of the polydimethylsiloxane liquid is unsatisfactory.